1. Field of the Invention:
This invention relates to a bus driver circuit for use in communications, and more particularly to a bus driver circuit for use in communications using a twisted-pair cable.
2. Description of Related Art:
Heretofore, there has been adopted a method which uses the twisted-pair cable in the field of communications between electronics control units (hereinafter referred to as ECUs) mounted on vehicles and especially such EUCs used in multiplex communications.
FIG. 8 and 9 of accompanying drawings are circuit diagrams of a driver circuit for use in communications using an existing twisted-pair cable.
FIG. 8 illustrates a system in which loads 10 and 12 independent of each other are driven by buses, respectively, and the busses are driven by means of a plurality of driver circuits, that is, the two driver circuits shown in FIG. 8. Each driver circuit has a closely similar configuration, and the structure of the driver circuits will be described with reference to a driver circuit 14 as an example.
The driver circuit 14 is comprised of two differential amplifiers 14a and 14c and two output transistors 14b and 14d. An output of the differential amplifier 14a is connected to the base of the output transistor 14b. The collector of the transistor 14b is connected to a source voltage Vcc, and the emitter of the same is connected to one wire of the twisted-pair cable. A current supply to the bus is effected by inputting a trapezoidal voltage waveform as shown in the drawing to the non-inverting input terminal of the differential amplifier 14a.
In the same manner, the differential amplifier 14c is connected to the output transistor 14d, and the bus draws a current for the other wire in the twisted-pair cable by inputting a trapezoidal voltage waveform having an inverted phase to the non-inverting input terminal of the differential amplifier 14c.
FIG. 9 illustrates a case where the bus driven by means of the plurality of driver circuits is connected to a resistor for current loop 16, and such a structure results in a current having a high symmetric property and prevents the occurrence of radio frequency noise because of the presence of the current loop resistor.
As a similar bus driver circuit using the twisted-pair cable, Japanese Patent Application No. Hei-2-16834 discloses a transceiver coupler, using the twisted-pair cable, for use in high speed networks. This transceiver coupler uses a transformer for transmitting a signal along the twisted-pair cable. The twisted-pair cable, along with a transformer coil and a load, constitutes a closed loop which supplies a current to the twisted-pair cable in equal amounts in both directions.
However, in the driver circuit for use in communications as shown in FIG. 8, since the loads 10 and 12 independent of each other are driven by the buses, respectively, the volt-ampere characteristic of each bus shows an asymmetric property, and radio frequency noise occurs as represented in the spectrum of FIG. 10.
Moreover, in the driver circuit shown in FIG. 9, although this driver circuit has a structure which causes less radio frequency noise, as show in FIG. 11, because of the driving of the current loop resistor as described above, if the CSMA/CD (carrier sense multiple access with collision detection) method is adopted as an access technique in multiplex communications, there is a probability that signals transferred from each driver circuit will cause collision, as well as an increase in the radio frequency noise as shown in FIG. 12. This is because the driving of the bus with a plurality of driver circuits causes a drastic change in the amount and direction of currents transmitted along the bus when the signals collide with each other, and because each driver circuit is subject to variations in voltage and amplitude controlled by the driver circuit and influences due to a potential difference between grounds.
The use of shielding wire to prevent the occurrence of radio frequency noise is widely known in the conventional art. This shielding wire, however, causes an increase in cost, and is hard to employ in vehicular applications.
Further, the structure including the transformer for driving the bus such as a transceiver coupler disclosed in the Japanese Patent Application No. Hei-2-16834 results in difficulties in integrating a circuit and reducing the size of the circuit, which is critical in mounting the circuit to the vehicle. In addition, in the closed loop of the signal line, a line breakage of one line disables the signal transmission, causing a degradation in reliability.